Electric power generating plants, industrial boilers, and other industrial processes generate particulates, acid gases and toxic materials that are frequently harmful to the environment. Particulate matter can remain suspended in the air for an extended period during which time the particulates present a potential health hazard. The particulates also tend to settle on surfaces such as buildings, machinery, or curtains, where they can cause unsightly blemishes or other problems. In addition, trace metals that often are harmful to humans and other species tend to concentrate on the fine particulates in a gas stream. Thus, it is important to remove particulates from an exhaust gas stream.
Acid gases, such as SO.sub.2 and SO.sub.x have been found to contribute to damaging acid rain. Technologies for control of acid gases such as spray dryers and scrubbers are well known in the art. However, such control systems are expensive and their installation requires significant amounts of space. Space constraints are especially troublesome in existing installations that must be retrofitted for acid gas removal.
Control of particulate emissions from industrial sources is accomplished largely by fabric filters and ESPs, with the greatest amount of particulate reduction being accomplished by ESPs. Current ESP technology operates upon the principle that particles are charged and then collected on the oppositely charged collector plates of an ESP. To accomplish this simultaneous charging and collection, a multiplicity of corona discharge electrodes are placed along the center line of a gas flow lane between a pair of grounded collector plates. A sufficiently high voltage is placed upon the corona discharge electrodes to cause the generation of a visible corona. The copious supply of ions formed by this corona charges particles in the gas, which are then attracted to the collecting plates by the electric field caused by the high voltage placed on the corona discharge electrodes relative to the grounded collector plates. Conventional ESP's are well documented by an abundant number of textbooks and other literature. Examples in the literature are: H, White, Industrial Electrostatic Precipitation, Addison-Wesley, Reading, Mass., 1963; and S. Oglesby and G. Nichols, Electrostatic Precipitation, Marcle-Dekker, N.Y., 1978.
Improvements in conventional ESP technology are disclosed in the patent literature. In the Environmental Protection Agency's ("EPA") U.S. Pat. No. 4,885,139 entitled Combined Electrostatic Precipitator and Acid Gas Removal System, which is hereby incorporated by reference, an ESP is disclosed in which a neutralizing slurry is sprayed into a chamber in the ESP so as to react with acid gases upstream of electrostatic precipitation. In the ESP disclosed in U.S. Pat. No. 4,885,139, the electrostatic collector section in a first section of the ESP is removed and replaced with a set of spray nozzles for injection of aqueous droplets of an acid gas neutralizing agent. The neutralizing agent is disclosed as being a slurry for calcium-based sorbents such as calcium carbonate or a clear solution with sodium-based sorbents such as sodium bicarbonate. The aqueous acid gas neutralizing agent is sprayed into the gas passing through the housing at a point upstream of the electrostatic collector section. U.S. Pat. No. 4,885,139 discloses that upon removing one electrostatic collector section to make room for neutralizing agent spray nozzles, it is necessary that the remaining electrostatic collector sections be upgraded with prechargers to restore the original particulate collection efficiency and to collect the injected sorbent.
EPA's U.S. Pat. No. 5,059,219 entitled Electroprecipitator with Alternating Charging and Short Collector Sections, which is hereby incorporated by reference, discloses a high efficiency ESP with multiple alternating charging and short collector sections. The ESP disclosed in U.S. Pat. No. 5,059,219 improves particulate removal efficiency by application of alternating charger and short collection sections. In an ESP with alternating charging and short collector sections, removal efficiency is improved by separating the functions of particulate charging and particulate collection.
In ESP systems with alternating charging and short collector sections, particulates passing through the ESP are charged in the charging section. The charger accomplishes this end by maximizing both the electric field and the current density present in the charger section. The high electric field makes it possible for the particulates to hold a relatively high charge. The high current density makes more charge available in the gas stream for charging particulates. The combination of a small diameter corona discharge electrode and large diameter grounded collector electrode in the charger section yields the desired electric field and current density.
When particulates passing through ESPs with alternating charging and short collector sections have high resistivities, the high current density in the charger section may result in a "back corona" phenomenon in the layer of particulates gathered on the grounded collector electrodes of the charging section. "Back corona" occurs when high resistivity particulates gathered on the collector electrode give rise to an increased electric field across the layer of particulates. This electric field can be sufficient to generate positive ions in the air spaces within the layer of particulates. Under "back corona" conditions, these positive ions tend to migrate back into the gas stream where they neutralize the negative charge on particulates, which in turn reduces the collection efficiency of the ESP. To overcome the "back corona" problem, the collector electrodes in the charging section of known ESP systems with alternating charging and collector sections are cooled, as for example be passing cooling water through the grounded electrodes of the charging section, so as to reduce the resistivity of particulates gathered on the collector electrodes of the charging section.
On the other hand, in the collector sections of known ESPs with alternating charging and collector sections, performance is optimized by maximizing the electric field while providing a minimal current density just sufficient to maintain electrostatic adherence of collected particulates to the grounded collector plates. The high electric field improves particulate collection because the force driving the particulates to the grounded collector plates of the collector section is proportional to the charge on the particles and the magnitude of the electric field. The current density is kept low to avoid "back corona" in the vicinity of the collector section grounded plates. When a small corona current flows from the corona discharge electrodes in the collector section to the grounded collector plates, an electric field develops in the layer of particulates on the collector plates. This field provides a clamping force that keeps particulates on the collector plates and prevents their reentrainment into the gas stream.
Due to the difference in desirable operating conditions between the charging and collector sections, the charging sections are conventionally placed a short distance upstream of the corresponding collector section so as to not interfere with the collection of particulates. However, this has proved structurally difficult because the collector electrodes of the charging and collector sections must be separately supported within the ESP and because the collected particulates must be separately removed, conventionally by mechanical rapping or scraping, from the grounded electrodes of the charging and grounded collector plates of the collector section. This structural arrangement frequently results in high maintenance and operating costs. In addition, separating the charging and collector sections tends to increase the size of the ESP.